1. Field
This disclosure relates generally to phase locked loops, and more specifically, to calibrating a dual port modulation phase locked loop.
2. Related Art
Phase locked loops (PLLs) with dual port modulation are widely used in radio frequency transceivers when the required frequency modulation far exceeds the PLL loop bandwidth. In a 2.4 GHz band, for example, most communication deployments/standards typically use a frequency deviation in the range of 10's-100's of kHz (i.e., Zigbee: ±500 kHz; Bluetooth low energy (BLE): up to ±250 kHz, etc.) at a modulation injection rate that is a multiple of the baseband symbol rate. For example, Zigbee has a chip rate of 2 MHz and BLE has a symbol rate of 1 MHz, but the input to the PLL may be applied at a reference clock derived rate (e.g., 16-48 MHz)). The wide modulation bandwidth necessitates the use of dual port modulation, which enables the PLL modulation rate to be independent of the PLL loop bandwidth, while requiring a very high stability of the channel frequency. The high stability requirement places an upper limit on the PLL loop bandwidth due to several noise sources in a PLL-based RF frequency generation.
In a dual port modulation PLL, the bulk of the modulation is directly injected into a voltage controlled oscillator (VCO) via a high port, where the modulation is scaled by the instantaneous VCO capacitance (or varactor control voltage) to frequency transfer function.
VCO modulation command to VCO output frequency deviation gain (Kmod) is a function of frequency (or tank capacitance) as well as process, voltage and temperature (PVT) variations. To make the VCO direct modulation immune to the non-linearity of the instantaneous VCO capacitance-to-frequency transfer function as well as to avoid accuracy variation over PVT variations, the high port modulation can be realized using a bank of very fine quantized digitally switchable varactors (with a capacitance in atto-Farads). In such an arrangement, the digital modulation can be realized accurately, if the frequency step size (or Kmod) of a high port varactor is precisely known, so that the number of varactors needed to realize a frequency modulation command can be accurately calculated. For modern connectivity standards, the transmitter modulation performance requirements imply better than 1-2% nominal accuracy on the estimation of the Kmod gain under nominal conditions, but needs to be better than 5% in the worst case conditions.